Lindsey D. Mayo

NF-κB activation by tumour necrosis factor requires the Akt serine–threonine kinase

Activation of the nuclear transcription factor NF-κB by inflammatory cytokines requires the successive action of NF-κB-inducing kinase (NIK) and an IκB-kinase (IKK) complex composed of IKKα and IKKβ. Here we show that the Akt serine–threonine kinase is involved in the activation of NF-κB by tumour necrosis factor (TNF). TNF activates phosphatidylinositol-3-OH kinase (PI(3)K) and its downstream target Akt (protein kinase B). Wortmannin (a PI(3)K inhibitor), dominant-negative PI(3)K or kinase-dead Akt inhibits TNF-mediated NF-κB activation. Constitutively active Akt induces NF-κB activity and this effect is blocked by dominant-negative NIK. Conversely, NIK activates NF-κB and this is blocked by kinase-dead Akt. Thus, both Akt and NIK are necessary for TNF activation of NF-κB. Akt mediates IKKα phosphorylation at threonine 23. Mutation of this amino acid blocks phosphorylation by Akt or TNF and activation of NF-κB. These findings indicate that Akt is part of a signalling pathway that is necessary for inducing key immune and inflammatory responses.

A phosphatidylinositol 3-kinase/Akt pathway promotes translocation of Mdm2 from the cytoplasm to the nucleus

The Mdm2 oncoprotein promotes cell survival and cell cycle progression by inhibiting the p53 tumor suppressor protein. To regulate p53, Mdm2 must gain nuclear entry, and the mechanism that induces this is now identified. Mitogen-induced activation of phosphatidylinositol 3-kinase (PI3-kinase) and its downstream target, the Akt/PKB serine-threonine kinase, results in phosphorylation of Mdm2 on serine 166 and serine 186. Phosphorylation on these sites is necessary for translocation of Mdm2 from the cytoplasm into the nucleus. Pharmacological blockade of PI3-kinase/Akt signaling or expression of dominant-negative PI3-kinase or Akt inhibits nuclear entry of Mdm2, increases cellular levels of p53, and augments p53 transcriptional activity. Expression of constitutively active Akt promotes nuclear entry of Mdm2, diminishes cellular levels of p53, and decreases p53 transcriptional activity. Mutation of the Akt phosphorylation sites in Mdm2 produces a mutant protein that is unable to enter the nucleus and increases p53 activity. The demonstration that PI3-kinase/Akt signaling affects Mdm2 localization provides insight into how this pathway, which is inappropriately activated in many malignancies, affects the function of p53.

A phosphatidylinositol 3-kinase/Akt/mTOR pathway mediates and PTEN antagonizes tumor necrosis factor inhibition of insulin signaling through insulin receptor substrate-1

Tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) by the insulin receptor permits this docking protein to interact with signaling proteins that promote insulin action. Serine phosphorylation uncouples IRS-1 from the insulin receptor, thereby inhibiting its tyrosine phosphorylation and insulin signaling. For this reason, there is great interest in identifying serine/threonine kinases for which IRS-1 is a substrate. Tumor necrosis factor (TNF) inhibited insulin-promoted tyrosine phosphorylation of IRS-1 and activated the Akt/protein kinase B serine-threonine kinase, a downstream target for phosphatidylinositol 3-kinase (PI 3-kinase). The effect of TNF on insulin-promoted tyrosine phosphorylation of IRS-1 was blocked by inhibition of PI 3-kinase and the PTEN tumor suppessor, which dephosphorylates the lipids that mediate PI 3-kinase functions, whereas constitutively active Akt impaired insulin-promoted IRS-1 tyrosine phosphorylation. Conversely, TNF inhibition of IRS-1 tyrosine phosphorylation was blocked by kinase dead Akt. Inhibition of IRS-1 tyrosine phosphorylation by TNF was blocked by rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), a downstream target of Akt. mTOR induced the serine phosphorylation of IRS-1 (Ser-636/639), and such phosphorylation was inhibited by rapamycin. These results suggest that TNF impairs insulin signaling through IRS-1 by activation of a PI 3-kinase/Akt/mTOR pathway, which is antagonized by PTEN.

The PTEN, Mdm2, p53 tumor suppressor–oncoprotein network

Oncoproteins and tumor-suppressor proteins regulate cell growth and viability. Recent observations show that phosphoinositide 3-kinase (PtdIns 3-kinase)-Akt signaling promotes the phosphorylation and movement of the Mdm2 oncoprotein into the nucleus, where it downregulates the p53 tumor-suppressor protein. The PTEN tumor suppressor protein inhibits activation of Akt and this restricts Mdm2 to the cytoplasm. Restriction of Mdm2 to the cytoplasm promotes p53 function and thereby sustains the sensitivity of cancer cells to chemotherapy. p53 acutely induces Mdm2, providing damaged cells the opportunity for repair, but subsequently induces PTEN, favoring the death of mutated or irrevocably damaged cells. Thus, oncoproteins and tumor suppressor proteins are networked to promote normal cell function and eliminate mutated cells.

PTEN Protects p53 from Mdm2 and Sensitizes Cancer Cells to Chemotherapy

The PTEN tumor suppressor protein inhibits phosphatidylinositol 3-kinase (PI3K)/Akt signaling that promotes translocation of Mdm2 into the nucleus. When restricted to the cytoplasm, Mdm2 is degraded. The ability of PTEN to inhibit the nuclear entry of Mdm2 increases the cellular content and transactivation of the p53 tumor suppressor protein. Retroviral transduction of PTEN into U87MG (PTEN null) glioblastoma cells increases p53 activity and expression of p53 target genes and induces cell cycle arrest. U87MG/PTEN glioblastoma cells are more sensitive than U87MG/PTEN null cells to death induced by etoposide, a chemotherapeutic agent that induces DNA damage. Previously, tumor suppressor proteins have been supposed to act individually to suppress cancers. Our results establish a direct connection between the activities of two major tumor suppressors and show that they act together to respond to stresses and malignancies. PTEN protects p53 from survival signals, permitting p53 to function as a guardian of the genome. By virtue of its capacity to protect p53, PTEN can sensitize tumor cells to chemotherapy that relies on p53 activity. p53 induces PTEN gene expression, and here it is shown that PTEN protects p53, indicating that a positive feedback loop may amplify the cellular response to stress, damage, and cancer.

Cell Type-specific Expression of the IκB Kinases Determines the Significance of Phosphatidylinositol 3-Kinase/Akt Signaling to NF-κB Activation

Phosphatidylinositol (PI) 3-kinase/Akt signaling activates NF-κB through pleiotropic, cell type-specific mechanisms. This study investigated the significance of PI 3-kinase/Akt signaling to tumor necrosis factor (TNF)-induced NF-κB activation in transformed, immortalized, and primary cells. Pharmacological inhibition of PI 3-kinase blocked TNF-induced NF-κB DNA binding in the 293 line of embryonic kidney cells, partially affected binding in MCF-7 breast cancer cells, HeLa and ME-180 cervical carcinoma cells, and NIH 3T3 cells but was without significant effect in H1299 and human umbilical vein endothelial cells, cell types in which TNF activated Akt. NF-κB is retained in the cytoplasm by inhibitory proteins, IκBs, which are phosphorylated and targeted for degradation by IκB kinases (IKKα and IKKβ). Expression and the ratios of IKKα and IKKβ, which homo- and heterodimerize, varied among cell types. Cells with a high proportion of IKKα (the IKK kinase activated by Akt) to IKKβ were most sensitive to PI 3-kinase inhibitors. Consequently, transient expression of IKKβ diminished the capacity of the inhibitors to block NF-κB DNA binding in 293 cells. Also, inhibitors of PI 3-kinase blocked NF-κB DNA binding in Ikkβ–/– but not Ikkα–/– or wild-type cells in which the ratio of IKKα to IKKβ is low. Thus, noncoordinate expression of IκB kinases plays a role in determining the cell type-specific role of Akt in NF-κB activation.

Levels of HdmX expression dictate the sensitivity of normal and transformed cells to Nutlin-3

Hdm2 and HdmX coordinately regulate the stability and function of p53. Each is overexpressed in subsets of many different types of malignancy, and most of these subsets maintain wild-type p53. Nutlins, newly discovered small-molecule inhibitors of the Hdm2-p53 interaction, offer a novel strategy for therapy of tumors with wild-type p53. We now show that Nutlin-3 efficiently induces apoptosis and diminishes long-term survival of human fibroblasts transformed in vitro by Hdm2 but not HdmX. The resistance of cells overexpressing HdmX to Nutlin-3 is due to its inability to disrupt the p53-HdmX interaction, resulting in continued suppression of p53 activity. Although HdmX overexpression yielded cells resistant to Nutlin-3, ablation of HdmX expression by short hairpin RNA sensitized tumor cells to Nutlin-3-mediated cell death or arrest. Furthermore, deletion of the COOH-terminal RING finger domain of HdmX completely reversed the resistance to Nutlin-3, probably reflecting the requirement of the RING finger for interaction with Hdm2. Thus, the relative abundance of Hdm2 and HdmX and the specificity of Nutlin-3 for Hdm2 influence the sensitivity of cells to p53-dependent apoptosis or arrest in response to Nutlin-3. Our findings establish Hdm2 and HdmX as independent therapeutic targets with respect to reactivating wild-type p53 as a means for cancer therapy.

Cell type specific expression of the IkB kinases determines the significance of PI 3-kinase/Akt signaling to NF-kB activation

Phosphatidylinositol (PI) 3-kinase/Akt signaling activates NF-κB through pleiotropic, cell type-specific mechanisms. This study investigated the significance of PI 3-kinase/Akt signaling to tumor necrosis factor (TNF)-induced NF-κB activation in transformed, immortalized, and primary cells. Pharmacological inhibition of PI 3-kinase blocked TNF-induced NF-κB DNA binding in the 293 line of embryonic kidney cells, partially affected binding in MCF-7 breast cancer cells, HeLa and ME-180 cervical carcinoma cells, and NIH 3T3 cells but was without significant effect in H1299 and human umbilical vein endothelial cells, cell types in which TNF activated Akt. NF-κB is retained in the cytoplasm by inhibitory proteins, IκBs, which are phosphorylated and targeted for degradation by IκB kinases (IKKα and IKKβ). Expression and the ratios of IKKα and IKKβ, which homo- and heterodimerize, varied among cell types. Cells with a high proportion of IKKα (the IKK kinase activated by Akt) to IKKβ were most sensitive to PI 3-kinase inhibitors. Consequently, transient expression of IKKβ diminished the capacity of the inhibitors to block NF-κB DNA binding in 293 cells. Also, inhibitors of PI 3-kinase blocked NF-κB DNA binding in Ikkβ–/– but not Ikkα–/– or wild-type cells in which the ratio of IKKα to IKKβ is low. Thus, noncoordinate expression of IκB kinases plays a role in determining the cell type-specific role of Akt in NF-κB activation.

Delayed activation of insulin-like growth factor-1 receptor/Src/MAPK/Egr-1 signaling regulates clusterin expression, a pro-survival factor.

Secretory clusterin protein (sCLU) is a general genotoxic stress-induced, pro-survival gene product implicated in aging, obesity, heart disease, and cancer. However, the regulatory signal transduction processes that control sCLU expression remain undefined. Here, we report that induction of sCLU is delayed, peaking 72 h after low doses of ionizing radiation, and is dependent on the up-regulation of insulin-like growth factor-1 as well as phosphorylation-dependent activation of its receptor (IGF-1 and IGF-1R, respectively). Activated IGF-1R then stimulates the downstream Src-Mek-Erk signal transduction cascade to ultimately transactivate the early growth response-1 (Egr-1) transcription factor, required for sCLU expression. Thus, ionizing radiation exposure causes stress-induced activation of IGF-1R-Src-Mek-Erk-Egr-1 signaling that regulates the sCLU pro-survival cascade pathway, important for radiation resistance in cancer therapy.

Ribosomal protein S3: A multi-functional protein that interacts with both p53 and MDM2 through its KH domain

The p53 protein responds to cellular stress and regulates genes involved in cell cycle, apoptosis, and DNA repair. Under normal conditions, p53 levels are kept low through MDM2-mediated ubiquitination and proteosomal degradation. In search for novel proteins that participate in this regulatory loop, we performed an MDM2 peptide pull-down assay and mass spectrometry to screen for potential interacting partners of MDM2. We identified ribosomal protein S3 (RPS3), whose interaction with MDM2, and notably p53, was further established by His and GST pull-down assays, fluorescence resonance energy transfer and an in situ proximity ligation assay. Additionally, in cells exposed to oxidative stress, p53 levels increased slightly over 24h, whereas MDM2 levels declined after 6h exposure, but rose over the next 18h of exposure. Conversely, in cells exposed to oxidative stress and harboring siRNA to knockdown RPS3 expression, decreased p53 levels and loss of the E3 ubiquitin ligase domain possessed by MDM2 were observed. DNA pull-down assays using a 7,8-dihydro-8-oxoguanine duplex oligonucleotide as a substrate found that RPS3 acted as a scaffold for the additional binding of MDM2 and p53, suggesting that RPS3 interacts with important proteins involved in maintaining genomic integrity.